The roof or Porro prism derives its name from the similarity of its configuration to that of a traditional A-roof house, albeit without eaves. Two faces of the Porro prism are orthogonal and oriented with respect to the rest of the prism so that they form an A-shaped roof over the remaining faces. The remaining faces form a rectangular box(without eaves but having a triangular portion meeting the apex of the roof on two of the four sides) attached to the two faces forming the A-roof. The face directly opposing the faces forming the roof is the input face of the Porro prism.
The Porro prism has the characteristic of retro-reflecting light in one axis. That is, light incident on the Porro prism will be directed back on itself, even as the prism is rotated by several degrees about a line running parallel to the roof crest or apex.
One application of the Porro prism is as an optical element in a laser resonator. A standard polarization output coupler (POC) has been built using at least two separate devices: the conventional Porro prism and a separate thin-film polarizer, or a pair of Porro prisms oriented in an opposing uncrossed geometry. A polarization output-coupling scheme for a laser resonator must either depolarize the circulating laser pulse in the cavity by rotating one of the roof crests of the two Porro prisms from a 90.degree. crossed configuration or use a separate polarizing-depolarizing device, such as a waveplate.